scripts: unset CONFIG_SITE

Some distributions (eg. openSUSE 12.1) systematically export
the CONFIG_SITE environment variable to point to a custom
script setting misc paths for ./configure.

This can, and does, break when cross-compiling for architectures
that are not supported by this script.

The simple workaround is to unset this variable.
NB: buildroot has a similar fix:
http://git.buildroot.org/buildroot/commit/?id=12c9f7dd6dee9c6029b4f9a12d6aac1516911ab4

Reported-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 21f4f28e60ec0342133086c9a51e7f0e5b181fb8)

--- trunk/src/acos.c	2009/12/08 16:49:43	733

+++ trunk/src/acos.c	2009/12/09 20:51:38	734

@@ -188,10 +188,7 @@

   else

     rnd_im = rnd_im == GMP_RNDU ? GMP_RNDD

       : rnd_im == GMP_RNDD ? GMP_RNDU

-#if MPFR_VERSION_MAJOR >= 3

-      : rnd_im == GMP_RNDA ? GMP_RNDZ

-#endif

-      : rnd_im;

+      : rnd_im; /* both RNDZ and RNDA map to themselves for -asin(z) */

   rnd1 = RNDC(GMP_RNDN, rnd_im);

   mpfr_init2 (pi_over_2, p);

   for (;;)